US7468609B2 - Switched suspended conductor and connection - Google Patents

Abstract

A probe assembly having a switch that selectively electrically connects, for example, either a Kelvin connection or a suspended guard element with the probe assembly.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No. 10/794,511, filed Mar. 5, 2005, now U.S. Pat No. 7,221,172; and which claims priority to U.S. Provisional App. No. 60/468,851, filed May 6, 2003.

BACKGROUND OF THE INVENTION

The present invention relates to a wafer assembly having a switch mechanism that, in one embodiment, selectively and alternately electrically connects a suspended element or connection with the probe assembly.

Testing of electrical devices is accomplished by routing a test signal through a probe assembly to selected contact points on a device under test (DUT). Measurement devices are then used to determine the voltages and/or currents generated in the DUT in response to that test signal.

When probing a DUT, it is highly desirable to protect the probe assembly from electrical interference that can result, for example, from dynamic electromagnetic fields present in the immediate vicinity of the probe assembly. To reduce this type of interference, probe assemblies commonly provide the test signal through a coaxial or triaxial cable so that the test signal may be guarded and/or shielded. In this arrangement, the center conductor of the coaxial or triaxial cable carries the test signal. A guard signal, preferably substantially identical to the test signal, or otherwise “follows” the test signal, is routed through either the outer conductor of a coaxial cable, or the middle conductor of a triaxial cable. If a triaxial cable is employed, a shield signal, which is typically instrumental “ground” or another relatively constant potential, may be provided through the outer conductor.

The DUT typically rests upon the top layer of a multilayer electrically conductive chuck where each layer is insulated from the others. The guard may be connected to the second layer of the chuck and, if desired, a conductive plate may laterally surround the chuck. The shield may be connected to the third layer of the chuck.

It is desirable to closely match the signals between the test signal and the guard. Because the shield is exposed to external interference along its outer periphery it will not ordinarily match precisely the guard signal it encloses. The resulting discrepancy between the shield and guard signals permits some interference to affect the guard signal. The modified guard signal in turn affects the test signal. Usually, the resulting interference is insignificant. However, when the test signal to the DUT is very small, even this amount of interference can be significant.

When small amounts of interference are significant, further reduction in the interference can be obtained by locating a suspended conductive plate over the DUT and connecting it to the guard signal. The suspended conductive plate defines a central opening so that the probe assembly may make electrical contact with the DUT. In this fashion, the DUT may be guarded from all sides by signals closely approximating that delivered to the DUT.

Another problem that occurs when testing a DUT with a probing assembly is that chuck and/or cable impedances can often inhibit accurate measurement of the voltages and/or currents present in response to the applied test signal. This problem is particularly prevalent with high-current test signals where the voltage drop along the cable and/or through the chuck can be significant.

The typical adaptation to this problem is to connect a second coaxial or triaxial cable to the probe assembly in the same manner as the first. In this two-cable configuration, commonly referred to as a Kelvin connection, one cable provides the “force” (e.g. a low impedance connection that primarily supplies a current) while the other cable provides a “sense” (e.g. a high impedance connection that primarily senses a voltage) used to measure the resulting voltages and/or currents present within the DUT. In a “true” Kelvin connection, both the force and the sense comprise independent paths from the test equipment to the DUT. Alternatively, both the force and the sense lines may be connected at a common interconnection point, that is as close to the DUT as possible, frequently referred to as a quasi-Kelvin connection.

Designing a probe assembly that provides both a suspended guard and a Kelvin connection has recently become problematical because of the total capacitance seen by the measurement equipment. Measurement equipment that is used in conjunction with the probe assembly only provides valid measurements so long as total circuit capacitance remains within specified parameters. In addition, testing equipment imposes capacitance restrictions between the force and guard, and the guard and shield, respectively.

Historically, probe assemblies containing both a suspended guard and a Kelvin connection could be designed within the capacitance limitations set by the testing equipment. In recent years, the industry has moved towards manufacturing individual electrical devices, such as silicon semiconductor components, on a larger-sized 12″ wafer. Any probe assembly that tests a DUT on such a wafer must therefore have a larger chuck, which adds capacitance to the probe assembly. The available capacitance permitted to be connected to the instrumentation that is not used by the chuck now must be rationed carefully. Both a suspended guard and a Kelvin connection add capacitance to the test circuit loop, and it has proven difficult to design a probe assembly that contains both elements, yet remains within the capacitance restrictions set by the measurement equipment.

What is desired, therefore, is a probe assembly that may accurately test electrical components and that includes a suspended guard and another connection.

The foregoing and other objectives, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 shows a cross-sectional view of a probe assembly that includes a Kelvin connection, a suspended guard element and a switching mechanism between the connection and suspended guard.

FIG. 2 shows a schematic for the switching mechanism shown inFIG. 1.

FIG. 3 shows a front view of an alternate embodiment of the top portion of the probe assembly shown inFIG. 1.

FIG. 4 shows a top view of the probe assembly shown inFIG. 1.

FIG. 5 shows a partially sectional side view of the probe and probe holder shown in the probe assembly inFIG. 1.

FIG. 6 shows a top sectional view taken along line6-6 inFIG. 3.

FIG. 7 shows a partially sectional bottom view of the probe assembly taken along line7-7 inFIG. 5.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

FIG. 1 shows aprobe station10 having aprobe assembly11 and amultilayered chuck assembly14 that may be used to probe anelectrical device12. Thechuck assembly14 includes atop stage16 that is electrically connected to acenter signal22. Thecenter signal22 may be the test signal return path to the signal delivered to theelectrical device12 when probed byprobe assembly11. To reduce electrical noise when theelectrical device12 is probed by theprobe assembly11, thechuck assembly14 also includes acenter layer18 that is electrically connected to aguard signal24, and abottom layer20 that is electrically connected to ashield signal26. Theguard signal24 may approximate the signal delivered to theelectrical device12 when probed. Some probe assemblies, like that disclosed in Peters et al., U.S. Pat. No. 6,002,263, utilize a variable shield signal which may be switched to other electric potentials. The manner in which center, guard, and shield signals are connected to their respective chuck stages is well known in industry, as disclosed for example, by Schwindt et al., U.S. Pat. No. 5,434,512.

Theguard signal24 may also be connected to a ring-shaped enclosure19 that surrounds thetop layer16 and themiddle layer18 of thechuck assembly14 along with theelectrical device12 being probed. In instances where further reduction in noise is desired, theprobe assembly10 includes aconductive guard element28 positioned over theelectrical device12 being probed. Theguard element28, along with thecenter signal22, theguard signal24, and theshield signal26 are electrically connected to aswitching mechanism40. It is to be understood that the suspendedguard28 may be merely vertically spaced apart from the DUT, whether above or below the chuck. Preferably, the suspendedguard28 is at least partially horizontally overlapping with the chuck and/or DUT supported by the chuck (but need not be). More preferably, the suspended guard is horizontally overlapping with the chuck and/or DUT supported by the chuck over a major portion of the chuck and/or DUT, respectively.

Theprobe assembly11 includes aprobe32 having aprobe end34 that contacts theelectrical device12 when probed. Theprobe32 separately routes a force signal provided along a firstelectrical path35 to aforce connection36 and a sense signal provided along a secondelectrical path37 from asense connection38 to theprobe end34. As seen inFIG. 1, both the force signal provided along theelectrical path35 and the sense signal provided along theelectrical path37 are electrically connected to theswitching mechanism40.

Referring toFIG. 2, theswitching mechanism40 receives thecenter signal22, theguard signal24, and theshield signal26 from test instrumentation (not shown). Theswitching mechanism40 routes signals to appropriate portions of theprobe assembly10. Referring also toFIG. 1, thecenter signal22 is routed to thetop layer16 of thechuck assembly14, and theforce connection36 of theprobe32 is routed through theforce connection35. Theguard signal24 is routed to thecenter layer18 of thechuck assembly14 and the ring-shapedenclosure19. Theshield signal26 is routed to thebottom layer20 of thechuck assembly14 and aninner shield enclosure60 surrounding thechuck assembly14.

Because of capacitance constraints imposed by test instrumentation, difficulties have been encountered when attempting to probe a device using a probe assembly containing both a Kelvin connection and a suspended guard element. A suspended guard element is normally required when a low-current test signal is employed along a single cable to make measurements, such as the sense connection of a Kelvin connection. The present inventors realized that the suspended guard element may not be necessary when the system is making Kelvin measurements because in contrast, a force connection typically uses a high-current test signal. Based upon this realization, the present inventors recognized the potential advantage of a probe station that includes a switch that selectively connects the force connection while disconnecting the suspended guard element when taking Kelvin (or quasi-Kelvin) measurements, and disconnects the sense connection while connecting the suspended guard element for non-Kelvin measurements. In other words, the switch selectively connects one of the cables (force or sense) for testing while disconnecting the suspended guard, and vice versa. In this manner, the additional capacitance of the suspended guard may be selectively removed from the signal path when its benefits are minimal. It is likewise to be understood that the switch mechanism may take the form of one or more switches that alone or in combination result in selectively adding or removing a signal path during testing while also correspondingly removing and adding, respectively, the suspended guard to the signal path. While the switching may be performed by activating a single switch(s) which performs the changes simultaneously (or substantially simultaneously) the changes may be manual, mechanical, and/or electrical. Further, the suspended plate need not necessarily be connected to guard, and as such may be connected to the signal, the shield, permitted to float, grounded, or otherwise. Moreover, the switching mechanism may likewise be useful for non-Kelvin testing. Further, the force, sense, or otherwise may be connected to the DUT, chuck, or otherwise, as desired.

As shown inFIG. 2, aswitch41 permits the probe station to be operated in two modes. In the first mode, theswitch41 electrically connects thecenter signal22 to thesense connection38 through thesense path37. In this first mode, theelectrical path30 to the suspendedguard28 is electrically disconnected from the test instrumentation. In the second mode, theswitch41 electrically connects theguard signal24 to the suspendedguard28 through theelectrical path30. In the second mode, thesense connection38 is electrically disconnected from the test instrumentation. Accordingly, thecenter signal22 may be used for testing purposes, if desired.

It is to be understood that more than one guard path or potential may be provided to the probe station, such as for example, a separate guard for the force and sense. Also, one or more test instruments may be used, collectively referred to as the test instrument. In addition, it may be advantageous under different circumstances to merely include a switch mechanism for the suspended guard to permit selectively including its respective capacitance in the signal path of the test instrument for any one or more channels.

Chuck Enclosure

Achuck assembly14 for supporting a test device is laterally positionable by achuck positioner assembly46 having an orthoganally arranged lateralX-axis positioner48 and Y-axis positioner50. The lateralX-axis positioner48 has a laterally extending positioning screw (not shown) driven by anelectric motor52. The X-axis positioner is partially enclosed by aconductive housing49 and optionally also by flexible pleated rubber boots (not shown) for accommodating positioning movements while preventing the entry of dirt particles. TheX-axis positioner48 selectively moves a Y-axis positioner50, oriented perpendicularly to theX-axis positioner48.

The lateral Y-axis positioner50 is constructed similarly to theX-axis positioner48, and includes an outerconductive housing51 with optional flexible pleated rubber boots (not shown). Theconductive housing51 is electrically connected to theconductive housing49 of the X-axis positioner. Amotor54 provides for adjustable movement of the Y-axis positioner.

A conventional Z-axis positioner56, having a conductiveouter housing57 electrically connected to theconductive housing51 of the Y-axis positioner50 is moveable along the Y-axis by the Y-axis positioner50. The Z-axis positioner includes respective internal electric motors and plunger assemblies (not shown) to effect vertical movement with respect to theX-axis positioner48 and the Y-axis positioner50. Such mechanisms are well known and used in existing probe assemblies, as shown for example, by Peters et al., U.S. Pat. No. 6,002,263.

The outerconductive enclosure42 is connected by alow impedance path58 directly to AC ground. Collectively, theouter enclosure42, aplaten44, and theconductive housings49,51, and57 cooperate to provide an electrically conductive outer shield enclosure which helps to insulate the chuck assembly from environmental noise sources located externally of theenclosure42 or internally within the positioner housings from motors, etc.

An electrically conductiveinner shield enclosure60, which also preferably acts as the probe station's environmental control enclosure not only for purposes of EMI shielding but also for purposes of maintaining a dry and/or dark environment, is mounted bydielectric spacers62 to the interior of theouter enclosure42 so as to be interposed between and insulated from theouter enclosure42 and thetop layer16 andcenter layer18 of thechuck assembly14. In this fashion, theouter shield components42,44,49,51, and57 protect theinner shield60 from external noise sources so that theinner shield60, in turn, can minimize noise-induced spurious currents affecting thetop layer16 andcenter layer18 of thechuck assembly14, and thereby maximize the accuracy of the test measurements.

Probe Assembly

Referring toFIGS. 1,3, and4, theprobe station10 includes the electrically conductiveouter enclosure42 including theconductive platen44 electrically connected thereto. Theplaten44 may be supported by a series ofjacks64 that selectively raise and lower the platen vertically by a small increment (approximately one-tenth of an inch). Theplaten44, when moved vertically by thejacks64, move independently of thechuck assembly14 and thechuck assembly positioner46.

Mounted atop theplaten44 are multiple individual probe positioners such as66 (only one of which is shown), each having an extendingmember68 to which is mounted aprobe holder70 which in turn supports arespective probe72 for contacting wafers and other test devices mounted atop thechuck assembly14. Theprobe positioner66 hasmicrometer adjustments74,76 and78 for adjusting the position of theprobe holder70, and thus theprobe72 along the X, Y and Z axes, respectively, relative to thechuck assembly14. The Z axis is exemplary of what is referred to herein loosely as the “axis of approach” between theprobe holder70 and thechuck assembly14, although directions of approach which are neither vertical not linear, along which the probe tip and wafer or other test device are brought into contact with each other, are also intended to be included within the meaning of the term “axis of approach.” A further micrometer adjustment80 adjustably tilts theprobe holder70 to adjust planarity of the probe with respect to the wafer or other test device supported by thechuck assembly14. As many as twelve (or more)individual probe positioners66, each supporting a respective probe, may be arranged on theplaten44 around thechuck assembly14 so as to converge radially toward thechuck assembly14 similarly to the spokes of a wheel. With such an arrangement, eachindividual positioner66 can independently adjust its respective probe in the X, Y and Z directions, while thejacks64 can be actuated to raise or lower theplaten44 and thus all of the positioners and their respective probes in unison.

The environment control enclosure is composed of anupper box portion82 rigidly attached to theplaten44 and alower box portion84 rigidly attached to the conductiveouter enclosure42 of thechuck assembly14. Both theupper box portion92 and thelower box portion84 are made of steel or other suitable electrically conductive material to provide EMI shielding. To accommodate the small vertical movement between the twobox portions82 and84 when thejacks64 are actuated to raise or lower theplaten44, an electrically conductiveresilient foam gasket86, preferably composed of silver or carbon-impregnated silicone, is interposed peripherally at their mating juncture at the front of the enclosure and between thelower box portion84 and theplaten44 so that an EMI, substantially hermetic, and light seal are all maintained despite relative vertical movement between the twobox portions82 and84. Even though theupper box portion82 is rigidly attached to theplaten44, a similar gasket (not shown) is preferably interposed between theportion82 and the top of the platen to maximize sealing.

With reference toFIG. 6, the top of theupper box portion82 comprises anoctagonal steel box88 having eight side panels such as89 and90 through which therespective probe positioners66 can penetrate movably. Atop the octagonal steel box88 a circular viewing aperture (not shown) is provided having a recessed circular transparent sealing window therein. Each panel comprises a hollow housing in which arespective sheet92 of resilient foam, which may be similar to the above-identified gasket material, is placed. Slits such as94 are partially cut vertically in the foam in alignment withslots96 formed in the inner and outer surfaces of each panel housing, through which arespective probe positioner66 can pass movably. The slitted foam permits X, Y and Z movement of each probe positioner, while maintaining the EMI, substantially hermetic, and light seal provided by the enclosure.

Referring toFIG. 4, theupper box portion82 of the environment control enclosure also includes a hingedsteel door98 that pivots outwardly about the pivot axis of ahinge100. The hinge biases the door downwardly toward the top of theupper box portion82 so that it forms a tight, overlapping slidingperipheral seal102 with the top of the upper box portion. When the door is open, and thechuck assembly14 is moved by thechuck positioner assembly46 beneath the door opening, thechuck assembly14 is accessible for loading and unloading.

With reference toFIGS. 5,6 and7, respective individuallymovable probes72 comprising pairs ofprobe elements104 are supported byrespective probe holders70 which in turn are supported by respective extendingmembers68 of different probe positioners such as66. Atop eachprobe positioner66 is ashield box106 having a pair oftriaxial connectors108,110 mounted thereon with respectivetriaxial cables112 entering each triaxial connector from a suitable test instrument as mentioned previously. Each triaxial connector includes a respectiveinner connector element114,116, anintermediate connector element118,120, and anouter connector element122,124 in concentric arrangement. Eachouter connector element122,124 terminates by connection with theshield box106. Conversely, theinner connector elements114,116, and theintermediate connector elements118,120, are connected, respectively, to the inner and outer conductors of a pair ofcoaxial cables126,128 which therefore are guarded cables. Eachcable126,128 terminates through a respectivecoaxial connector130,132 with arespective probe element104 having acenter conductor136 surrounded by aguard138. In order to provide adequate shielding for thecoaxial cables126,128, especially in the region outside of theoctagonal box88, an electrically-conductive shield tube140 is provided around thecables126,128 and electrically connected through theshield box106 with theouter connector element122,124 of the respectivetriaxial connectors108,110. Theshield tube140 passes through the same slit in the sheet offoam92 as does theprobe positioner66. Thus, each individually movable probe has not only its own separate individually movable probe holder but also its own individually movable shield for its guarded coaxial cables, which shield is movable in unison with the probe holder independently of the movement of any other probe holder by any other positioning mechanism. This feature is particularly advantageous because such individually movable probes are normally not equipped for both shielded and guarded connections, which deficiency is solved by the described structure. Accordingly, theprobes72 are capable of being used with the same guarding and Kelvin connection techniques in a ready-to-use manner as is thechuck assembly14, consistently with full shielding despite the individual positioning capability of eachprobe72.

The terms and expressions which have been employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims which follow.

Claims (18)

I claim:

1. An assembly comprising:

(a) a support suitable to support a device-under-test;

(b) a first member defining a first electrical path to said device-under-test;

(c) a second member defining a second electrical path to said device-under-test;

(d) a conductive member vertically spaced apart from at least a portion of said device-under-test; and

(e) a switch mechanism that alternately electrically connects said second member to a test instrument while electrically isolating said conductive member from said test instrument, and electrically isolates said second member from said test instrument while electrically connects said conductive member to said test instrument.

2. The assembly ofclaim 1 wherein said conductive member is electrically connected to a guard potential.

3. The assembly ofclaim 1 wherein at least one of said first member and said second member is a cable.

4. The assembly ofclaim 1 wherein said first member and said second member form a Kelvin measurement.

5. The assembly ofclaim 1 wherein said first member and said second member form a quasi-Kelvin measurement.

6. The assembly ofclaim 4 wherein said second member is a sense connection.

7. The assembly ofclaim 5 wherein said second member is a sense connection.

8. The assembly ofclaim 1 wherein said isolating and said electrically connects is substantially simultaneous.

9. The assembly ofclaim 1 wherein at least a portion of said conductive member at least partially overlaps said support.

10. An assembly comprising:

(a) a support suitable to support a device-under-test;

(b) a first member defining a first electrical path to said device-under-test;

(c) a conductive member vertically spaced apart from at least a portion of said device-under-test; and

(d) a switch mechanism that alternately electrically connects said first member to a test instrument while electrically isolating said conductive member from said test instrument, and electrically isolates said first member from said test instrument while electrically connects said conductive member to said test instrument.

11. The assembly ofclaim 10 wherein said conductive member is electrically connected to a guard potential.

12. The assembly ofclaim 10 wherein said first member is a cable.

13. The assembly ofclaim 10 wherein said first member is part of a Kelvin measurement.

14. The assembly ofclaim 10 wherein said first member is a part of a quasi-Kelvin measurement.

15. The assembly ofclaim 13 wherein said first member is a sense connection.

16. The assembly ofclaim 14 wherein said first member is a sense connection.

17. The assembly ofclaim 10 wherein said isolating and said electrically connects is substantially simultaneous.

18. The assembly ofclaim 10 wherein at least a portion of said conductive member at least partially overlaps said support.

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